Early events in coxsackievirus cell interaction include attachment, eclipse, and uncoating of virions as essential processes leading to productive infection. Having developed a procedure for solubilizing the group B coxsackievirus receptor from HeLa cell plasma membranes, we intend to purify and characterize these receptors as well as develop a method to quantitate the amount of virus-binding activity of soluble receptor preparations. We plan to apply these techniques to determine the comparative distribution of the Group B coxsackievirus receptor in newborn and adult mice. These latter studies will provide data to evaluate the role of receptors as determinants of virus tropism in pathogenesis. We have provided evidence for the grouping of picornaviruses into subgroups which correspond to their original classification based on disease production. Data will be sought to extend the groupings of picornaviruses according to their cellular receptor specificity. Attention will be given to identifying the virion attaching protein of coxsackievirus B3 which binds to receptors. To extend our knowledge of the structure of this virus we plan to determine the virus group and type specific antigens associated with the capsid polypetides by ELISA and RIA methods. Receptor-mediated virus eclipse produces heat labile subviral particles termed "A" particles. We plan to characterize further the naturally occurring stabilizer of coxsackievirus B3 "A" particles in HeLa cell plasma membranes and to determine the biological significance of each. In a related area of these receptor studies, experiments are proposed to determine how differentiating myogenic cells in culture acquire susceptibility to infection by coxsackieviruses of Group A.